Telomeric reciprocal recombination as a possible mechanism for antigenic variation in trypanosomes.

In African trypanosomes, antigenic variation is achieved through differential gene activation, with one antigen gene being expressed at a time among a large collection of antigen-specific sequences. Transcription of the antigen gene always takes place in a telomere, but different telomeres can alternatively act as the expression site. Telomeric antigen genes can be expressed without apparent DNA rearrangement, but they can also, like non-telomeric genes, have access to the telomeric expression site through a duplicative transposition mechanism resembling gene conversion. We report here that, as previously suggested, telomeric genes may use another route to be activated. This mechanism of gene activation is by reciprocal crossing-over upstream from the gene, in the so-called 'barren' region. This allows the antigen gene to be placed in the previously activated telomere, while inactivating the formerly expressed gene by recombination into a silent environment. At least for the telomeric antigen gene described here, three possible activation mechanisms coexist.